The present disclosure relates generally to ignition systems for gas burners and more particularly to ignition systems having electrical resistive hot surface igniters.
Currently, gas cooktops consist of two to eight individual gas burners mounted atop a metallic or ceramic glass cooktop surface. Generally, the burners will consist of a cap and a main burner body, where exhaust gas ports are placed around its periphery. Gas is supplied through user actuable valves that individually control flow to a respective burner. The gas is then directed through gas tubing to an orifice where the gas flow is exhausted at flow rates sufficient to entrain enough air into the gas flow stream to permit combustion at the exhaust ports of the burners. The gas at each burner is ignited at some point near one or several of the exhaust ports. According to United States regulatory requirements, ignition devices for the gas burners must successfully ignite the gas within four (4) seconds of turning the valve to a corresponding ignition point. Generally, cooktop appliances or gas range appliances in the United States and throughout the world either use a spark igniter or a standing pilot system to ignite the gas/air combination exhausted out of the exhaust ports. The standing pilot is a low flow, continuous flame that stays lit even when the system is not in use.
Spark igniters generally only function when a control of a gas cooking appliance is set to a certain position, i.e. at the ignition setpoint on the gas valve knob. Spark igniter systems generally energize all igniters regardless of the specific gas knob being activated. A variation of a spark igniter system that is commonly used on higher end products consists of spark igniter system with a flame sense technology that permits the spark igniter to fire at any non-off control knob position if flame is not sensed. There are several flame sense technologies in practice including those that use temperature sensing devices and those that detect ground to igniter voltage changes when the flame is no longer present. Both of these flame sense technologies are hampered by occasional reliability issues where a false loss of flame is sensed and the igniters fire off sparks when not necessary. Because of the unnecessary activation of the spark igniters, when for example, a burner flame is falsely determined to be lost or out, and because the burner flame sensing is hidden to the consumer, there is often a mistrust of the technology and a perception of not being trustworthy in properly evaluating loss of flame.
In some gas oven applications, hot surface, electrical resistive igniters have replaced pilot lights and spark igniters. Because of their fairly large thermal mass, typical hot surface igniters are relatively slow to reach ignition temperatures and thus require a delay between the user turning on the oven and the opening of the gas control valve feeding fuel to the gas burners inside the oven. It is fairly common for ovens to require a delay on the order of thirty (30) to sixty (60) seconds to allow the surface igniter to heat up to an auto-ignition temperature (e.g. about 700 degrees centigrade for natural gas). While this approach is acceptable to the consumer oven applications, such an extended delay would create a perception to the same consumer of unsafe operation for a cooktop application. Recent advances in low mass, highly conductive ceramic, hot surface igniters using such base materials as silicon nitride, silicon carbide, and other comparable inorganic compounds have led to the development of hot surface igniters that can reach ignition temperatures well within the four (4) second threshold, while still meeting reasonable expectations for a long service life, more attainable. However, the development of a fast responding system using hot surface igniters for appliance applications has been hindered by complexity, lack of reliability, and/or high cost. Typically, microcomputers have been used to control the heating of the hot surface igniter. In one example, an ignition system for a gas burner uses a control algorithm based on an alternating current (AC) modulated signal where a second voltage is applied to the hot surface igniter for maintaining a temperature lower than the fuel ignition temperature. Here the steady state voltage with the igniter below the fuel ignition temperature is intended to permit a longer igniter life cycle. These igniters that are maintained at a steady state below the fuel ignition temperature are generally a silicon nitride igniter with a tungsten filament that is prone to aging.
In another example the microcomputer controls the igniter so that the igniter is rapidly heated via control of the AC power supply to attain ignition temperature and then subsequently reduced from the initial power levels to maintain ignition temperature based on a learning routine. In other examples, the level of AC power to the igniter is based on the determined value of AC voltage available to energize the igniter and on the determined value of the igniter resistance. In still other examples, power is modulated to the igniter by trimming alternating current cycles using, for example, triacs. The main disadvantages of such microcomputer based approaches include a fairly high level of complexity and cost, the potential of software based decisions acting inappropriately for a safety critical system, and, in the case of the AC modulated solutions, a risk of failing due to excessive amounts of power being fed into the igniter. In many applications, there is a requirement that flame sensing technology must be employed concurrently with the hot surface technology to enable a sufficiently long use life. This approach, however, is contrary to research that shows many consumers would prefer to see a continuously glowing igniter as it is perceived to be a more reliable ignition source and to make it easy to detect that igniter is not working properly.
It would be advantageous to control a low voltage DC powered electrical resistive igniter without a microprocessor so that a flame is ignited within a predetermined time period where the igniter is reliable throughout a projected life of a cooktop on which it is installed.